Young Earth was a boiling sphere of liquid rock, while Theia, which crashed into it, had had time to cool and solidify.
Similarly, Earth’s interior spewed into space, forming a cloud of cosmic dust and debris. From this, the Moon gradually coalesced. This hypothesis complements the standard model of the giant impact.
Analysis of lunar soil using light analysis (pioneered by Joseph von Fraunhofer) revealed that the atomic composition of the Moon and Earth is nearly identical. Decades of mystery were solved: due to the difference in fluidity, it was mostly Earth material that escaped into space. The most unexpected outcome: the Moon rocks brought back by the Apollo missions are frozen splashes of our planet. Holding them, you’re touching a shard of ancient Earth.
That same impact likely tilted Earth’s axis, giving us the seasons. So we have spring and autumn to thank that ancient catastrophe for.